Project Summary. The Gram-negative bacterium Acinetobacter baumannii is a leading cause of infections in both the hospital setting and in military personnel injured during wartime. The goal of this Research Career Scientist Award is to continue addressing the mechanisms that regulate virulence and ??lactam resistance in A. baumannii. A high-frequency switch that regulates the transition between virulent (opaque) and avirulent (translucent) forms of A. baumannii will be studied using a combination of genetic and biochemical approaches. The role of a putative bistable transcriptional regulator in the opaque to translucent virulence switch will be investigated by genetic and transcriptomic approaches. A second area of research is focused on the regulation of intrinsic ?-lactam resistance in A. baumannii. Our work has identified a novel gene in A. baumannii that is required for intrinsic ??lactam resistance. Inactivation of this gene sensitizes A. baumannii to ??lactams and also confers a prominent cell division defect. This provides the first link between defects in cell division and decreased intrinsic ?-lactam resistance and suggests novel interventions that target cell division may sensitize cells to ?-lactams. These research projects are highly collaborative and involve VA investigators at both the Atlanta VAMC and the Louis Stokes VAMC in Cleveland, OH. This project will continue to foster these collaborations and allow for the development of new projects. This project will also have an important training component, where mentoring for junior VA investigators will be provided. Lastly, student and postdoctoral training will continue to be an integral component of this research plan, with the goal of developing the next generation of VA scientists by mentoring a CDA-2 awarded project.